1. Field of the Invention
The present invention relates to a light emitting element utilizing electroluminescence and a light emitting device provided with the light emitting element. Further, the present invention relates to a method for manufacturing the light emitting element.
2. Description of the Related Art
In recent years, a light emitting element using a light emitting organic compound has been actively researched and developed. A basic structure of the light emitting element is that in which a layer containing a light emitting organic compound (light emitting layer) is sandwiched between a pair of electrodes. By applying voltage to this element, electrons and holes are separately injected from the pair of electrodes into the light emitting layer, and current flows. Then, recombination of these carriers (the electrons and holes) makes the light emitting organic compound in an excited state and emit light when the excited state returns to the ground state. Owing to such a mechanism, such a light emitting element is referred to as a current-excitation light emitting element.
Note that an excited state of an organic compound includes a singlet excited state and a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
A great advantage of such a light emitting element is that the light emitting element can be manufactured to be thin and lightweight, since the light emitting element is generally formed of an organic thin film having a thickness of 0.1 μm or so. In addition, extremely high response speed is another advantage since period between carrier injection and light emission is approximately microseconds or less. These characteristics are considered suitable for a flat panel display element.
Such a light emitting element is formed in a film shape. Thus, surface emission can be easily obtained by forming a large area element. This characteristic is hard to be obtained by a point light source typified by an incandescent lamp or an LED or a line light source typified by a fluorescent lamp. Therefore, the above described light emitting element has high utility value also as a surface light source applicable to lighting or the like.
Thus, the current-excitation light emitting element using the light emitting organic compound is expected to be applied to a light emitting device, lighting, or the like. However, there are still many issues. Improvement in luminous efficiency is one of the issues.
For instance, when a display device for color display is manufactured using a light emitting element, there is a method for obtaining three primary colors of red, green, and blue using a light emitting element of white color and a color filter. In this case, a red color filter transmits only red light in light emission from the light emitting element, and light with other wavelengths is blocked. At the same time, green and blue color filters transmit only green and blue light, respectively, and light with other wavelengths is blocked. Namely, when the light passes through the color filter, about two thirds of luminescence from the light emitting element are not taken out and are wasted. Therefore, a white light emitting element with much higher luminous efficiency is required.